1. Field of the Invention
The present invention relates to an optical communication system in which an optical beam is modulated in accordance with an electrical RF signal in a transmitter, and the modulated optical beam is then transmitted to a remote receiver.
2. The Background Art
It is known that an optical beam can be modulated in accordance with an electrical RF signal using an optical modulator such as a Mach-Zehnder modulator. Such optical modulators have a nonlinear relationship between the input electrical signal and the output optical signal, but this nonlinear relationship can be mitigated by applying an electrical bias signal to the optical modulator to bias the optical modulator to a quadrature point at which the relationship between the input electrical signal and the output optical signal is approximately linear. Typically, the optical modulator is maintained with a fixed bias at the quadrature point by controlling the electrical bias signal in accordance with the output optical signal.
A problem with biasing an optical modulator at the quadrature point is that the input electrical radio-frequency (RF) signal normally only introduces a small modulation into the output optical signal unless the electrical RF signal is amplified. Such a small modulation is undesirable because a high proportion of the optical signal carries no information. Amplifying the input electrical RF signal increases the proportion of the optical signal which carries information, but the electrical RF amplification is undesirable in that it introduces noise, degrades linearity and introduces amplitude and phase variations across the frequency band of the transmitted RF signals.
In the article “Increased Linear Dynamic Range by Low Biasing the Mach-Zehnder Modulator” by Mark L. Farwell et al., IEEE Photonics Technology Letters, Vol. 5, No. 7, July 1993, pp. 779-782, a “low biasing” scheme is reported in which an optical bias below the conventional quadrature bias is used. Such an arrangement produces a relatively high modulation depth of the optical signal without any amplification of the input electrical RF signal. For single octave applications, a theoretical increase in linear dynamic range under certain conditions is predicted. In the article “Gain Optimization by Modulator-Bias Control in Radio-Over-Fiber Links” by Marco M. Sisto et al., Journal of Lightwave Technology Vol. 24, No. 12, December 2006, pp. 4974-4982, the combination of a low-biased Mach-Zehnder modulator and an Erbium-Doped Fiber Amplifier (EDFA) is discussed.